Fluorine polymers are always used for applications which require special properties, such as low surface tension or high resistance to chemicals or which impose extreme requirements on ageing resistance. For many applications, the fluorine polymers are crosslinked during or after molding, as for example where they are used in the rubber field. In the case of unmodified fluorine elastomers, i.e. elastomers which do not contain any special reactive sites, these crosslinking reactions require drastic conditions so that crosslinking is difficult to control and damage can be caused to the fluorine polymer. To avoid this, reactive sites, so-called cure sites, are introduced into the polymer chains so that the polymers can be selectively crosslinked under milder conditions. The cure sites in question are, for example, bromine or iodine substituents which are introduced into the fluorine polymer either by copolymerizing bromine- and/or iodine-containing vinyl compounds in small quantities with the fluorine monomers (U.S. Pat. No. 3,351,619; U.S. Pat. No. 4,035,565; U.S. Pat. No. 4,214,060; DE-OS 3 715 210) or by carrying out the polymerization in the presence of saturated compounds containing iodine or bromine and iodide (DE-OS 2 815 187; DE-OS 3 710 818). Bromine- and/or iodine-modified fluorine polymers such as these can be radically crosslinked, for example by peroxidic compounds, in the presence of a co-crosslinking agent. Triallyl cyanurate or triallyl isocyanurate may be used as the co-crosslinker (Rubber Chem. Technol. 55 (1982), 1004).
Compared with bisphenol-crosslinked types for example, these peroxide-crosslinked fluorine rubbers show considerably improved resistance to bases, nucleophiles and oxidation (Kautschuk und Gummi, Kunststoffe 38 (1985), 471).
However, the bromine- or iodine-containing fluorine polymers have the disadvantage that low molecular weight alkyl bromides or iodides can be formed during the radical crosslinking reaction, evaporating during the crosslinking reaction and thus impairing the processing and performance properties.
Accordingly, there was a need for fluorine polymers which can be crosslinked radically, for example by peroxides, under mild conditions and which do not have any of the weaknesses mentioned above. Basically, these desirable properties could be achieved by incorporation of functional monomers, i.e. monomers containing reactive double bonds, if controlled incorporation leaving the double bonds intact were possible.
However, if fluorine monomers are polymerized in accordance with the prior art in the presence of polyunsaturated monomers, such as perfluorodivinyl ether (U.S. Pat. No. 3,851,018), the polymers are crosslinked to a considerable extent during the actual copolymerization, which is undesirable for many applications.
In addition to a number of monomers, non-fluorinated conjugated dienes, such as butadiene, isoprene or chloroprene, are mentioned as potential comonomers in EP 360 575 A2. However, these monomers either inhibit the polymerization of the fluorine monomers or lead to crosslinked products, so that these monomers are also unsuitable for the production of the desired uncrosslinked fluorine polymers containing lateral double bonds.
It has now been found that fluorine-containing vinyl compounds of the type mentioned below and unconjugated dienes of the type mentioned further below can be copolymerized in such a way that substantially uncrosslinked copolymers are formed which contain reactive double bonds as side groups and which can be radically crosslinked (vulcanized) under mild conditions. Unconjugated dienes are understood by the expert to be dienes in which the two C--C double bonds are separated by at least one sp.sup.3 -hybridized C atom.